Calendar decreed by Pope Gregory XIII in 1582 to reform the Julian calendar.The Gregorian calendar dates back to 1582 and is the calendar used by the vast majority of countries in the world. It comprises 365 1/4 days. A normal year comprises 7 months of 31 days, 4 months of 30 days and one month of 30 or 28 days (leap year). The calendar is a solar calendar based on the cycle of the sun. The Gregorian calendar was originally the Julian calendar (used in Orthodox Christian religions). According to the Julian calendar, a year was 11 minutes and 14 seconds longer than the solar year. A difference of 10 days had accumulated by 1582. Pope Gregory XIII therefore modified the calendar by establishing the following system: a leap year occurs when the year number is divisible by 4. An exception is made for secular years ending in "00" as these years are only leap years if divisible by 400. Hence the years 1700, 1800 and 1900 are common years whereas the year 2000 is a leap year. The average length of a year is 365.2425 days. It is very close to that of a tropical year. The Gregorian calendar spread throughout Western Europe and its former colonies. Key dates:Great Britain adopted the Gregorian calendar in 1752.Japan abandoned the Chinese calendar in favour of the Gregorian calendar in 1873.China adopted the Gregorian calendar as its official calendar in 1912.Russia adopted it in 1918.Greece in 1923.